1. Field of the Invention
The present invention relates to a method of producing a powder, the powder and a multilayered ceramic capacitor using the same.
2. Description of the Related Art
Recently, improvement in performance and miniaturization of electronic devices is rapidly advanced, along with the progress of electronic parts such as IC and LSI. In accordance therewith, improvement in performance and reduction in size in capacitors, being an electronic part, and there exists a rapidly increasing demand for multilayered ceramic capacitors. For example, what terminal electrodes is formed on a capacitor element consisting of dielectric layers and internal electrodes alternately laminated is known as such a multilayered ceramic capacitor. Along with the demand for improvement in performance and miniaturization of the multilayered ceramic capacitors, there is a demand for improvement in electrical characteristics of its dielectric material, i.e., dielectric powder such as barium titanate.
The dielectric layer of a multilayered ceramic capacitor is required to satisfy, for example, a requirement for X7R specified in the standards of the U.S. Electronic Industries Association (EIA Standard) that the capacity temperature change rate in a temperature range of −55° C. to 125° C. should be within ±15% from that at a standard temperature of 25° C. For satisfying the requirement for X7R, proposed is a method of preparing particles of a dielectric powder having a core/shell structure. The core/shell structure is a structure of particle having a core portion inside the particle and a shell portion covering the core portion. For example, an additive containing various additional components such as magnesium and yttrium is added to the barium titanate powder and the resulting mixture is sintered, for forming the core/shell structure. In such a case, the particle contains pure barium titanate as a composition of the core portion and a solid solution of barium titanate and magnesium or yttrium as a composition of the shell portion. Presence of a shell portion of a dielectric material containing additional components as a shell portion reduces the fluctuation in specific dielectric constant in the temperature range at which the multilayered ceramic capacitor or the like is used, and allow production of a multilayered ceramic capacitor having an extended lifetime. Accordingly, there exists a need for particles different in composition between the inside the particle and the surface thereof, like those having a core/shell structure.
However, when a capacitor is sintered using a dielectric powder prepared by conventional methods, a solid solution of magnesium or yttrium added as the shell component becomes uneven in many cases and consequently generating heterogeneous phases in the composition after sintering. In addition, when a multilayered ceramic capacitor is prepared, excessive additional components, which are not sufficiently incorporated into the solid solution with the principal component, remained on the grain boundary, are reacted with the internal electrodes and therefore caused a problem of short circuiting of the internal electrodes.
To overcome such a problem, methods of producing a dielectric powder having a core/shell structure using a solid-phase method are disclosed (e.g., Japanese Patent Application Laid-Open (JP-A) No. 10-310469).